1. Field of the Invention
The present invention relates to a system for measuring a distance to an object to be measured and more particularly to an active distance measurement system favorably applied to various types of cameras.
2. Related Background Art
Such an active distance measurement system applied to cameras generally includes an infrared-emitting diode (IRED) for emitting an infrared beam toward an object to be measured, and a position sensitive detector (PSD) for receiving the object-reflected infrared beam. The signal output from the PSD is a signal responsive to a position where the object-reflected infrared beam is received. A signal processing and arithmetic unit determines a distance to the object 30 from this signal. Because a large error may occur at once measurement, averaging of multiple pieces of distance information is generally performed to obtain more accurate distance information.
FIG. 9 shows a circuit diagram illustrating a configuration of an integrating unit used for obtaining the average of the multiple pieces of distance information in the distance measurement system. This integrating unit 16 comprises a switch 1, an integrating capacitor 2, a switch 3, a constant current source 4, an operational amplifier 5, a switch 6, a reference power source 7, and a comparator 8. The negative input terminal of the operational amplifier 5 is connected through the switch 1 to the output terminal of an arithmetic unit 15 and grounded through the integrating capacitor 2. Furthermore the negative input terminal of the operational amplifier 5 is connected through the switch 3 to the constant current source 4, and connected through the switch 6 to the output terminal of the operational amplifier 5. Also, the positive input terminal of the operational amplifier 5 is connected to the reference power source 7, which provides a reference voltage VREF. The comparator 8 is connected to the junction between the negative terminal of the operational amplifier 5 and the integrating capacitor 2 and compares the potential of the junction and the reference voltage VREF to find out which is higher. The comparator 8 outputs a signal corresponding to the comparison results. A central processing unit (CPU) 19 receives the signal output from the comparator 8 and controls the on-off operation of the switches 1, 3 and 6.
As an example of the distance measurement system using such an integrating unit 16 is a distance measurement system mounted in a camera. When a shutter release button is half- or partially-depressed after powering on the camera, the CPU 19 turns on the switch 6 to charge the integrating capacitor 2. As the result, the integrating capacitor 2 is charged to the reference voltage VREF provided by the reference power source 7. After the charging up, the switch 6 is turned off and retained in such a state.
Then, the IRED emits infrared pulses and the switch 1 is turned on. As a result, output signals (distance information) from the arithmetic unit 15 are input into the integrating capacitor 2 as negative voltages. The voltage across the integrating capacitor 2 decrementally changes step by step in value corresponding to each distance measurement information. This is called a xe2x80x9cfirst integratingxe2x80x9d.
After the predetermined number (e.g., 256) of negative voltage inputs (discharges) into the integrating capacitor 2 are completed, the switch 1 is turned off and the switch 3 is turned on in response to control signals from the CPU, whereby the integrating capacitor 2 is charged at a fixed speed defined by the power rating of the constant current source 4. This is called a xe2x80x9csecond integratingxe2x80x9d.
All the while of the second integrating, the comparator 8 compares the voltage level of the integrating capacitor 2 and the reference voltage VREF. If the comparator 8 estimates that they are coincident with each other then the comparator 8 turns the switch 3 off to stop charging the integrating capacitor 2, i.e. finish the second integrating. The CPU 19 counts a charging time of capacitor 2 (length of time spent in the second integrating). As the charging speed by the constant current source 4 is uniform, the sum of the signal voltages input into the integrating capacitor 2 during the first integrating can be determined from the aforementioned charging time of capacitor 2. The distance to the object can be determined based on the resultant sum. On the basis of the obtained distance to the object, the CPU 19 controls a driving of lens to focus properly on the object to be imaged.
With such a distance measurement system, as the operating characteristics of the signal processing and arithmetic unit vary with temperature, not only the discharging characteristics of the integrating capacitor 2, but also the process of second integrating. Thus, there is a problem that distance measurement accuracy may be highly dependent on temperatures.
It order to remove such a problem, one suggested solvent is using a transformation that compensate for changes in temperatures. Such a transformation may be used for determining the distance to the object or driving lens. However, since parameters used in the transformation are real numbers, it is necessary to provide a large capacity of memory device (e.g., an; EEPROM; electrically erasable and programmable ROM) storing such parameters therein. The transformation also loads on the CPU.
In order to solve the above-mentioned problems, it is an object of the present invention to provide a distance measurement system, which can measure a distance with high accuracy and with low CPU load regardless of changes in temperature.
To achieve the above object, a distance measurement system according to the present invention is a active distance measurement system comprising: (1) a light source for emitting a predetermined series of light pulses toward the object to be measured; (2) a PSD for receiving object-reflected light pulses and outputting signals each corresponding to the position where the object-reflected pulse is received; (3) an arithmetic unit for outputting signals each corresponding to the distance to the object in response to signals output from said PSD; (4) an integrating capacitor set to a first reference voltage before emitting said series of light pulses, said capacitor being charged or discharged in response to said signals output from said arithmetic unit; (5) a distance detection unit for detecting the distance to the object based on a voltage of said capacitor present after emitting said series of light pulses; (6) a temperature sensing unit for measuring an ambient temperature; and (7) a control unit for controlling said distance measurement system so as to adjust the charging and discharging operation of said capacitor in response to the ambient temperature measured by said temperature sensing unit.
According to the present invention, a series of light pulses are emitted toward the object to be measured from the light source. These pulses are reflected by the object and thus object-reflected pulses are received by the PSD. The position where the object-reflected pulse is received changes responsive to the distance to the object. The PSD outputs a signal dependent on this light-receiving position. The arithmetic unit determines the distance to the object by the output signal from the PSD with a given calculation and outputs the distance signal. The arithmetic unit outputs distance signals each corresponding to a result of distance measurement by single pulse emission. The integrating unit is charged or discharged in response to each distance signal so that the distance signals are integrated. The distance detecting unit outputs the average of distance measurement values. At this conjuncture, the charging and discharging operation of the integrating capacitor is adjusted based on the temperature measured by the temperature sensing unit, so that the integrated results of the integrating capacitor become independent of the temperature, but only dependent on the distance to the object. This results in accurate measurements.
Furthermore, the control unit favorably adjusts one or both of the number and periods of the charges or discharges of the integrating capacitor. In the case of the number of charges or discharges being fixed, each period of charge or discharge may be adjusted, or some periods of charge or discharge may be adjusted.
Especially, it is preferable to accomplish the adjustment by adjusting the number of charges or discharges of the integrating capacitor referred to a predetermined number. The period of charge or discharge of the integrating capacitor is preferably adjusted below the predetermined reference period. This is effective for the removal of standing-light component. In these cases, the adjustments may be made easier and a capacity of storing program may be diminished.
The control unit in the distance measurement system according to the present invention may adjust the output signal from the arithmetic unit response to the ambient temperature measured by the temperature sensing unit to adjust the charging and discharging operation of the integrating capacitor.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.